This invention relates generally to a new and improved cryogenic regenerator (sometimes referred to merely as a regenerator), and more particularly is an improvement of the multiple channel regenerator section formed with rolled, corrugated and smooth foils enclosed within tubular walls as disclosed in FIG. 10 of U.S. Pat. No. 4,619,112, issued Oct. 28, 1986 to S. A. Colgate and assigned to Colgate Thermodynamics Company and entitled Stirling Cycle Machine. In addition, this invention relates to a new and improved process of manufacturing the cryogenic regenerator of the present invention.
As known to those skilled in the art, a cryogenic regenerator typically interconnects two compression-expansion chambers and conducts a working fluid (e.g. a gas such as helium) between the chambers. As is further known to those skilled in the art, upon the regenerator being interconnected between the chambers, one end may be at, for example, 300.degree. K., and the other end may be at, for example, 70.degree. K.; thus, a regenerator may be said to have hot and cold ends or relatively hot and relatively cold ends.
As is still further known to those skilled in the art, a major source of thermodynamic efficiency in the Stirling Cycle Machine and in other similar regenerative cryocoolers is the regenerator. This is especially true under high frequency; the stationary regenerator must exchange heat with the working fluid as the fluid passes back and forth through the regenerator at some frequency. To accomplish this in an efficient manner, a number of competing parameters must be dealt with.
An effective cryogenic regenerator must have sufficient heat capacity relative to the total enthalpy change of the working fluid. The regenerator must have a large surface area in contact with the working fluid so that only a small temperature difference exists between the cryogenic regenerator and the working fluid. Thermal conduction between the hot and cold ends of the cryogenic regenerator should be small compared to refrigeration (heating). The pressure drop across the regenerator should be small enough to keep viscous losses small. The void (dead volume in the regenerator) fraction should be small for high regenerator effectiveness and good heat transfer must be present between the working fluid and the regenerator (i.e. the material of which the regenerator is constructed) for high regenerator effectiveness.
The cryogenic regenerator must, therefore, make use of the proper geometry and materials of which it is constructed. While the geometry of the cryogenic regenerator is controlled by the designer, the optimum material for all temperature ranges may not exist, that is a single material may not be optimum for all such temperature ranges. An effective regenerator would optimize the paramaters noted above and should, desirably, be easy and inexpensive to construct. As is still further known to those skilled in the art, most commercially available materials suitable for cryogenic regenerator construction fail to meet one or more of these requirements.
Further, the typical cryogenic regenerator, e.g. the regenerator disclosed in the Stirling patent noted above, must effectively relate five variables. This poses a difficult design problem since many of the variables are conflicting. These variables are:
1. Non-ideal heat exchange between the working fluid (e.g. helium or nitrogen, etc.) and the regenerator material.
2. Extra work and frictional heat due to viscosity which causes pressure drop in the regenerator.
3. Loss because of dead volume of gas within the regenerator that does not expand or contract during the cycle, thus limiting the cycle compression ratio.
4. The departure from isothermality, because of the mass of the regenerator material.
5. Thermal conduction in the direction of primar heat flow, i.e., the axial or longitudinal direction of the regenerator.
Optimization of these five variables leads to a channel cryogenic regenerator with the working fluid moving with laminar flow through the channels. Variables 1 through 3 above deal mainly with the geometry of the regenerator, while variables 4 and 5 are primarily concerned with the materials used to construct the regenerator. If the regenerator is to span a large temperature range, the prior art (e.g. above-noted Stirling patent) teaches that the regenerator may be divided into sections--each individual section must be optimized with respect to heat capacity and longitudinal and axial thermal conductivity (material specific) and channel flow area, area of working fluid, e.g. gas, contact and length (geometry specific). For each section comprising such multi-section cryogenic regenerator, both geometry and material must change as the section operating temperature changes. The material of which each cryogenic regenerator section is constructed must have a high heat capacity so that its temperature change is small during the passage or conducting of the working fluid therethrough. Further, it has been discovered that the amount of material for the cryogenic regenerator section and its configuration must be consistent with low thermal conduction in the direction of fluid flow. It has been generally found that even alloys such as stainless steel will have too much conduction in the direction of flow while insulators such as plastics must be too thin in order to allow thermal penetration. It has been found that the manufacture of such prior art multi-channel cryogenic regenerator is difficult and expensive; particularly, the prior art spirally rolled cryogenic regenerator shown in FIG. 10 of the above-noted Colgate patent is manufactured, as illustrated in FIG. 11 thereof, from a plurality of individual members, i.e. foils 1005 and 1006, with foil 1005 being corrugated, and with the foils in turn supported by a tubular wall 1004. Accordingly, there exists a need in the art for a cryogenic regenerator which is easy and inexpensive to manufacture and which preferably is manufactured, not from a plurality of individual members which must be assembled, but instead which is integrally formed or formed from a single material, a single composite material.
As is known to those skilled in the art, the working fluid, e.g. gas, increases in density as it is conducted through the regenerator channels from the hot end to the cold end of the regenerator, and, as is still further known, the depth or transverse cross-sectional area of the prior art regenerator channels is constant along the length of the regenerator. Thus, there exists an undesirable mismatch between the increasing density of the working fluid as it is conducted through the regenerator channels from the hot end to the cold end of the regenerator and the depth or transverse cross-sectional area of the prior art regenerator channels. This mismatch undesirably increases the gas volume present in the regenerator and undesirably increases the pressure drop across the regenerator. Accordingly, there exists a need in the art for a better match between the density of the working fluid and the depth or transverse cross-sectional area of the regenerator channels. It has been discovered, and in accordance with the teachings of the present invention, that by continuously decreasing the depth or continuously decreasing the transverse cross-sectional area of the channels from the hot end to the cold end of the regenerator a better match is provided between the working fluid and the depth or transverse cross-sectional area of the channels whereby both the volume of working fluid present in the regenerator and the pressure drop across the regenerator are both desirably decreased.
As noted above, further, good heat transfer must be present between the regenerator, i.e. the material of which the regenerator is constructed, and the working fluid being conducted therethrough for high generator effectiveness. It has been discovered, and in accordance with the further teachings of the present invention, that by continuously increasing the radial thermal conductivity of the material of which the regenerator is made from the hot end to the cold end increased heat transfer is desirably provided between the regenerator and the working fluid; of course, such continuous increase in axial thermal conductivity undesirably increases the axial or longitudinal thermal heat leak of the regenerator from the hot end to the cold end but it has been discovered that this is an acceptable compromise which is more than offset by the increased heat transfer.
Accordingly, it has been found that there exists a need in the art for new and improved multiple channel cryogenic regenerator which optimizes the above-noted five variables (preferably varying the optimization of these variables continuously over the length of the regenerator thereby avoiding the need for multi-sections) and which is easy and inexpensive to manufacture; it has been found that there exists a corollary need with and for a new and improved process of manufacturing such cryogenic regenerator.